Steering wheel

ABSTRACT

A steering wheel is produced from injection molding of a foamed thermoplastic elastomer. The thermoplastic foam material can produce desirable characteristics for both the rim and the airbag cover. The rim of the steering wheel has good wear characteristics and a targeted compressibility or “feel” while the airbag cover has the sought-after strength across a large temperature range and exhibits very little compliance. The thermoplastic foam material is also beneficial because it has a non-olefinic foam carrier, such as polyvinyl acetate, which does not degrade the adhesive used to attach a cushion and/or a outside wrap to the steering wheel rim.

This application is a continuation in part of U.S. Ser. No. 09/358,812,filed Jul. 22, 1999, U.S. Pat. No. 6,164,691.

FIELD OF THE INVENTION

This invention relates to a steering wheel armature coated with athermoplastic foamed material.

BACKGROUND OF THE INVENTION

Steering wheels in general comprise a metal armature that is enclosed inan appropriate covering material such as wood, elastomeric substance, ora combination of the two. With the development of technology ofattaching the steering wheel with the airbag module juxtaposed to thesteering column (i.e. U.S. Pat. No. 5,897,132 to Papandreou and U.S.Pat. No. 5,692,769 to Scharboneau et al.), there became an impetus fordeveloping a single material composite steering wheel/airbag cover. Thedifficulty with this feat is the physical properties required of amaterial for the airbag cover are considerably different from thoserequired of a material in the rim of a steering wheel. The obstacle ofproducing an integral steering wheel (rim, airbag cover, and spokes aremolded from same material) has partially been overcome by utilizingreaction injection molded (RIM) polyurethane. This process produces afinal surface that has very good wear properties while being compliantto the touch and relatively low in mass. These properties are highlydesirable for a steering wheel rim and, in fact many standard(non-integrated) steering wheels are produced using this process. Whilethis process has also been used extensively for airbag covers in thepast, the use of RIM urethane is not nearly as desirable for thisapplication due to the tendency of urethane to fragment during airbagdeployment. U.S. Pat. No. 5,692,769 teaches that to overcome thisdrawback a tough, non-fragmenting material such as a substrate or ascrim is added beneath the airbag cover. Thus, it is inherent in thisstep that a substrate must be produced prior to the final urethanemolding phase and must be introduced into the polyurethane molding tool,by some method, prior to introduction of the polyurethane. This stepconsumes time during the manufacturing process and adds complexity andcost to the molding due to the necessity of adding attachment featuresto secure the substrate during the RIM process.

The RIM polyurethane process has additional disadvantages. To promoteproper filling of the mold cavity many air vents are employed and thetool is closed in such a way that some additional venting occurs at themeeting point of the two halves of the mold. Invariably somepolyurethane material fills the vent areas during the molding processwith the result that the molded part must later be trimmed to remove theexcess material. During the trimming process it is easy to inadvertentlydamage the final surface of the product rendering it unacceptable foruse.

An additional concern with the reaction injection molding process is theduration of the cycle. This cycle is typically two to three times longerin duration than the cycle associated with a standard injection moldingprocess for a similar part.

To overcome the numerous disadvantages stated above it would bedesirable to produce the final surface of the integrated steering wheelusing an alternate process. The chief alternative used today to producesteering wheels and airbag covers is standard injection molding.Typically, automotive steering wheels are fabricated using thermoplasticpolyvinyl chloride (PVC) based resins while airbag covers are fabricatedfrom high-performance engineered resins known generally as thermoplasticelastomers. These two materials, as used in the subject applications,have very different physical properties. PVC is formulated for good wearcharacteristics and a targeted compressibility or “feel.” However,steering wheel rims made of PVC with plasticizers may encounter adhesionproblems with paints or outside covers on the rim due to the migrationof the plasticizers to the surface. The thermoplastic elastomers arechosen for chiefly their strength across a large temperature range andexhibit very little compliance. It is generally recognized thatimproving the compliance characteristics of a thermoplastic produces anadverse effect on strength and vice versa.

SUMMARY OF THE INVENTION

In order to produce an integrated steering wheel in a single-shotinjection molding process a thermoplastic material must be found thatwill satisfy the dissimilar performance requirements of the steeringwheel rim and airbag cover portions.

In a first embodiment of this invention, a single thermoplastic foammaterial is injected into a steering wheel mold to produce an integralsteering wheel; the composite steering wheel/airbag cover has acontinuous external surface molded from a single thermoplastic material.The rim portion and the spoke portions act as support structures for thesingle thermoplastic foam material.

The rim of the steering wheel has good wear characteristics and atargeted compressibility or “feel” while the airbag cover has thesought-after strength across a large temperature range and exhibits verylittle compressibility. The airbag cover has the necessary tear strengthso that a reinforcing member or substrate does not have to be addedunderneath the airbag cover.

In a second embodiment of this invention, a cushion and outside wrap,such as leather, cover a steering wheel rim made of thermoplastic foammaterial. The thermoplastic foam material with a non-olefinic foamcarrier, such as polyvinyl acetate (PVA), does not degrade the adhesiveused to attach a cushion and/or an outside wrap covering the steeringwheel rim. The configuration of the wrap and cushion result in asteering wheel with the required feel and effective adhesion to thesteering wheel rim.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of the metal armature of thesteering wheel.

FIG. 2 is a pictorial representation of the integral steering wheel withthe thermoplastic resin coating molded to the armature shown in FIG. 1.

FIG. 3 is a cross section taken along line 3—3 of FIG. 2

FIG. 4 is an enlarged cross section of the steering wheel rim takenalong line 4—4 of FIG. 2.

FIG. 5 is a pictorial representation of a non-integral steering wheelwith the thermoplastic resin coating molded to the armature shown inFIG. 1.

FIG. 6 is a cross section taken along line 6—6 of FIG. 5.

FIG. 7 is an enlarged cross section of a wrapped steering wheel rimtaken along line 7—7 of FIG. 5.

FIG. 8 is a graph showing the peel strength of PVC and TPEE plaques heataged at 85° C.

DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of a steering wheel armature 1, which istypically made of a suitable metal such as magnesium. The armature hasthree main components: a rim 8, a hub 2, and spokes 9, which correspondto like numbered components of the finished steering wheel shown in FIG.2. It is understood that the armature shown in FIG. 1 and the finishedsteering wheel shown in FIG. 2 are only exemplary of such items and thatthe structure, appearance, and dimensions may be varied in accordancewith good engineering practices without deviating from the scope of thepresent invention.

A steering wheel 10 as shown in FIG. 2 has three main components, eachrequiring different physical characteristics. The airbag cover 7 alignedwith and spaced apart from the hub 2 of the armature, needs to havephysical characteristics that will facilitate the proper deployment ofan airbag 11 (installed between the hub and the airbag cover), while thesteering wheel rim 8 needs to have physical characteristics thatdemonstrate good wear characteristics and cause the steering wheel to besufficiently compressible to impart a comfortable feeling to the driver.The spokes 9 are the supporting components of the steering wheel thatcouple the rim of the steering wheel to the airbag cover and armaturehub. The rim portion and the spoke portions are the support structuresfor the thermoplastic foam material. The composite steering wheel/airbagcover has a continuous external surface molded from a singlethermoplastic material. FIG. 3 is a cross section of FIG. 2 taken alongline 3—3 showing the different components of the steering wheel. The hub2 is the portion of the steering wheel that connects to the steeringcolumn securing the steering wheel to the car. Above the hub is aphantom representation of an airbag module containing both the airbag 11and the inflator; the airbag cover 7 is depicted above the module. Thesteering wheel according to a certain preferred embodiment of thisinvention is comprised of a single foamed thermoplastic material moldedto the armature of the steering wheel. over thermoset materials(i.e.polyurethane) such as having greatly reduced molding cycle time,and being able to recycle scrap. Thermoplastic resins significantlyreduce manufacturing costs because an auxiliary support member,sometimes referred to as a scrim, is not required for the airbag coverportion of a steering wheel. The prior art does not identify a singlethermoplastic material that will yield the necessary physicalcharacteristics for both the rim portion of a steering wheel and theportion of the steering wheel that will function as an airbag cover.

A number of thermoplastic materials were evaluated for possible use inmolding a composite steering wheel/airbag cover. A series of tests wereperformed on specimens of several different thermoplastic resins (nofoaming agent was added) in an attempt to discover a resin that canprovide the required physical properties in molding a steering wheel.Resins such as Santoprene 101-65 (obtained from Advanced ElastomerSystems), DYM 7038 (obtained from Dupont Engineering Polymers), and PL280 (obtained from DSM Engineering Plastics) were eliminated ascandidates for molding a composite steering wheel/airbag cover forvarious reasons. Santoprene 101-65 produced unsatisfactory results inthe instrumented impact testing, which looked at the maximum load of theairbag cover. DYM 7038 did not yield desirable molding characteristics,and PL 280 did not produce consistent results on a number of tests.Specimens of Arnitel EM 400 yielded the best overall results in thefollowing described tests: instrumented impact testing, deployment testsat 25° C., 85° C., and −40° C., and various molding tests. Arnitel EM400 was obtained from DSM Engineering Plastics located at 26877Northwestern Highway, Suite 410, Southfield, Mich. USA 48034 (Thecorporate headquarters is in the Netherlands). The exact details of itschemical composition are not known; however, it is known that EM 400 isa flexible thermoplastic elastomer (an elastomer is flexible if amonomer has a large alkyl group). Table 1 provides some data on ArnitelEM 400 polyester elastomer from published DSM sales literature.

TABLE 1 Categorical property Specific property Property value PhysicalHardness Shore D  38 Mechanical Izod notched No break impact strength(23° C.) Thermal Vicat softening 140 point (10N at 120° C.)

Arnitel EM 400 had the necessary prerequisite physical, mechanical, andthermal properties to substantiate further research. It is understood bythose skilled in the art that other thermoplastic elastomers, such aspolypropylene, may be used for the steering wheel. The next step inevaluating the EM400 thermoplastic material was to study its propertiesin the foam state. The addition of a foaming agent to the copolymerchanges the inherent properties of the resin. The foaming agent reducesthe specific gravity or the density of the polymer because of thepresence of voids or pores embedded in the resin. A foam steering wheelhas an improved softness and uses less resin than a steering wheel madefrom a non-foam resin. In a preferred embodiment of the presentinvention, the density reduction of the thermoplastic material for therim 8 should be at least 10% when the foaming agent is added. One of thefoaming agents used is identified as Clariant CLM70 manufactured by DSM(manufacturing company of DSM Arnitel 400). The Clariant CLM70 foamingagent consists of a foam, a color, and an olefin wax carrier, but exactdetails on the chemical composition of the foaming agent are not knownbecause DSM has a proprietary right to the chemical. The preferredfoaming agent for this application is identified as Clariant PRA0698418manufactured by Clariant Chemical. The Clariant PRA0698418 foaming agentdoes not contain an olefin carrier, but has a polyvinyl acetate (PVA)carrier. The PVA carrier is preferred when adhesion of material onto thesubstrate material is an issue. The chemical make up of the foamingagent Clariant PPA0698418 is not known because Clariant Chemical has aproprietary right to the chemical. The foaming agent generates a gas ata specific temperature that is consistent with the melting point of thethermoplastic resin. As the resin melts, the foaming agent generates agas that expands and creates the foam.

The thermoplastic resin without the foaming agent has a homogenous cellstructure; however the addition of the foaming agent results in theformation of three layers which can be viewed in FIG. 4: the internalskin 14, the external skin 15, and the underlying core 16. The internaland external skins have a greater density than the underlying core.These skins allow for better adhesion to the armature and a cover.

EXPERIMENT 1: ASCERTAINING TEAR STRENGTH, HARDNESS, AND PERCENT DENSITYREDUCTION

Plaques of EM 400 were studied to gather preliminary informationconcerning the foamed product. Tests were performed on ⅛ inch thick and¼ inch thick plaques of non-foam and foamed Arnitel EM 400 with foamingagent Clariant CLM70. Although these tests were run using the ClariantCLM70 foaming agent, it is believed that the results would be similarfor the foaming agent Clariant PRA0698418. The different thicknessplaques have remarkably different physical characteristics because athinner section plaque can cool more quickly and thus does not allow thethermoplastic material the chance to expand. The ⅛″ plaque is not thickenough to allow for sufficient expansion and thus a definitive corelayer is not present. The ¼″ plaque is thick enough to allow forexpansion and thus two distinct layers are formed: the external skin andthe underlying core. These two different thickness plaques were chosenbecause they simulate the rim and the airbag cover of the integralsteering wheel. The ⅛ inch plaques serve as a model for the airbag coverwhile the thicker ¼ inch plaques serve as a model for the rim portion ofthe steering wheel.

Tear tests were performed on the plaques at three different temperaturesto ensure sufficient strength of the foamed resin at extremetemperatures. The samples were subjected to tear testing per ASTM D624using an Instron 5568 universal testing machine. Table 2 provides thetear strength at three different temperatures for foam EM 400 and forthe non-foam resin.

TABLE 2 Plaque −40° C. thickness 85° C. (lbf/in) Ambient (lbf/in) EM 400¼″ 145 ± 10.6 272 ± 12.6 1176 ± 59.7  (foam) EM 400 ¼″ 392 ± 2.9 506 ±5.9 748 ± 54.1 (non-foam) EM 400 ⅛″ 349 ± 0.45 444 ± 11.8 806 ± 89.3(foam) EM 400 ⅛″ 349 ± 0.5 451 ± 4.8 769 ± 35.5 (non-foam)

It has been determined that the tear strength for an airbag coverportion of a steering wheel at 85° C. needs to be above 100 pounds perlinear inch and greater than 150 pounds per linear inch at roomtemperature. According to these established requirements, the tearstrengths obtained for the plaques had sufficient magnitude to besatisfactory for use in producing a composite steering wheel/airbagcover.

For the second test, a hardness test, the samples were subjected toDurometer Shore A hardness testing per ASTM D2240. The hardness test wasemployed in an attempt to quantify the “feel” or compressibility of thethermoplastic material. Table 3 provides the hardness values for foamand non-foam EM 400.

TABLE 3 Plaque Average Shore thickness A Hardness EM 400 ¼″ 91.9 ± 0.20(foam) EM 400 ¼″ 91.0 ± 0.89 (non-foam) EM 400 92.8 ± 1.03 ⅛″ (foam) EM400 ⅛″ 93.1 ± 0.58 (non-foam)

For the Shore A hardness test, the associated hardness of the substanceincreases as the measured value increases. The foam ¼″ thickness plaquesshould have a smaller measured value for the hardness than the non-foam¼″ thickness plaques because the expansion of the thermoplastic materialshould result in a “softer” more compressible material. However, themeasured values were similar due to the limitations of the Shore Ahardness test. This method of hardness determination has inherentlimitations due to the fact that the caliper only measures the hardnessof the surface of the plaque without taking into account the overallhardness of the plaque (the surface along with the underlying core). Asubjective test of manually touching or compressing the plaques wasperformed to find out if the desired “hardness” was achieved. The foam⅛″ plaques were determined to be a little too hard, and thus minorvariations of EM 400 are being evaluated to uncover a derivative of EM400, which is slightly softer.

The density of the foam provides great insight about its physicalproperties. If the density reduction (the difference in density of thenon-foam EM 400 and the foam EM 400) is small, then the final productdoes not have the desirable softness and there is negligible reductionin resin quantity. On the other hand, if the density reduction is great,then there will be a large amount of voids and this will result in thefinal product being deformable. Table 4 provides the average percentdensity reduction from fifteen ¼″ plaques and fifteen ⅛″ plaques.

TABLE 4 Average Plaque initial Foamed Percent Specific thickness weight(g) Weight (g) reduction gravity EN 400 76.60 55.8 27.21 0.83 ¼″ EN 40040.35 38.72 4.04 1.09 ⅛″

The change in density is very trivial for the ⅛″ plaque, which isfavorable because the airbag cover needs to have a high tear strength.On the other hand, the ¼″ plaques demonstrated a significant percentreduction (27.21), which is desirable because the rim portion of thesteering wheel needs to be soft to the touch. In the preferredembodiment, the density reduction of the steering wheel rim materialshould be at least 10%.

EXPERIMENT 2: PAINT STUDIES

The ⅛″ and ¼″ foamed plaques were sent to Sherwin Williams, a paintcompany, to make certain that the foamed resin plaques could be easilypainted. Testing performed at Sherwin Williams showed that plaques ofboth thicknesses were paintable without adhesion promoter. Both samplesof EM400 passed initial adhesion and adhesion after 96 hour waterimmersion. While this is not considered exhaustive testing for adhesion,it is a very good indicator that the material is paintable withoutadhesion promoter, which reduces the costs of manufacturing.

EXPERIMENT 3: PHYSICAL PROPERTIES OF THE STEERING WHEEL RIM

In a certain preferred embodiment the steering wheel is a one componentsystem; the foamed Arnitel EM 400 is molded to form the rim 8, thespokes 9, and the airbag cover 7 portions of the steering wheel 10.Other foamed thermoplastic elastomers could be used, such as foamedpolypropylene. The foamed thermoplastic elastomer may also be used for astandard, non-integral, steering wheel/airbag. For example the foamedEM400 may be used only for the rim and spokes. The process for moldingthe steering wheel is a single shot injection technique. Moldingparameters can be modified to alter the skin thickness and overalldensity. These include mold temperature, injection time, and shot sizeamong others. For the molding of most steering wheels the optimal moldtemperature range is from 120° F. to 190° F., and the optimal injectionrate is from two to four seconds per cavity. The shot size, the volumeof material to fill the mold, will vary depending on the size of thesteering wheel.

The different portions of the steering wheel have different densityreductions. The airbag cover has a density reduction of between 5-20%while the rim and spokes have a density reduction of up to 40%. Thesmaller density reduction for the airbag cover contributes to the coverhaving the desirable strength to withstand fragmentation duringdeployment.

Utilizing the molding parameters discussed previously, two prototypesteering wheels were molded and then subjected to skin and foam cellexamination. One steering wheel was injected with 4% (by weight) foamingagent while the other steering wheel was injected with 2% (by weight)foaming agent. The thermoplastic resin obtained for this experiment wasin the concentrate form with the foaming agent preblended with theresin. The Arnitel EM 400 was injected by means of a single gate intothe steering wheel mold. In Table 5, the first column containsinformation indicating the percentage of foaming agent added to thethermoplastic resin. Also the first column contains informationconcerning the location of the samples removed from the steering wheel.At three discrete locations on the rim of the steering wheel, a sectionwas removed. FIG. 2 shows the three locations: bottom 4, middle 5 andtop 6. On each section of the steering wheel, four samples were examinedwhich can easily be seen from FIG. 4: grip 17 (undulation 3 for fingerpositioning), 90° from grip 18, 180° from grip 19, and 270° from grip20. The sections are circular and hence the degrees indicate thedistance around the circle that the samples were removed. In columnsthree through five of Table 5, values are presented of the skinthickness as well as the foam thickness. For labeling purposes of thecompletely symmetrical steering wheel, the area of the steering wheelwhere the thermoplastic foam material is injected is designated as thebottom of the steering wheel. The third column contains data fromsamples located on the opposite end of the injection gate. The fourthcolumn contains data from a point halfway around the rim of the steeringwheel while the fifth column contains data from the area where the foamwas injected.

TABLE 5 (column 2) Location around (column 3) (column 4) (column 5)(col 1) Wheel> Top Middle Bottom Sample Location around the Thickness(mm) Thickness (mm) Thickness (mm) ID Rim ↓ Skin Foam Skin Foam SkinFoam 2% Grip-FIG. 4 #20 No clear delineation 1.7 5.5 0.63 6.4 betweenthe skin/ foam was noted 2% 90° CW from grip Same as above 1.3 1.1 0.686.1 FIG. 4-#19 2% 180° CW from grip Same as above 1.5 3.6 .92 7.7 FIG.4-#18 2% 270° CW from grip Same as above 1.6 3.6 .85 2.6 FIG. 4-#17 4%Grip Same as above 2.1 5.9 1.5 5.7 FIG. 4-#20 4% 90° CW from grip Sameas above 1.9 0.6 .95 6.0 FIG. 4-#19 4% 180° CW from grip Same as above1.3 3.6 .95 7.6 FIG. 4-#18 4% 270° CW from grip Same as above 1.8 3.3.73 3.0 FIG. 4-#17

The results from Table 5 were obtained by using a Mitutoyo Digimaticvernier caliper. Due to the general lack of clear delineation betweenthe skin and foam, the findings are visual approximations. The skinmeasurements were representative of the outer skin only. The datareveals that regardless of the location of the sample on a section onthe top portion of the steering wheel, there is no clear skin layerpresent. The bottom (at gate) region exhibited smaller voids moreconcentrated near the inner skin area. The level of voids at the middlearea of sample 2% was minimal, with more voids noted in the same area insample 4%. The top region of the steering wheel (end of fill) exhibitedlarge voids within the cross section (ascertained by using Javelincamera with a Vivitar macrozoom lens system), some as large as the totalcross-section. Thus utilizing a one gate system for injecting the foamedEM 400 yielded unsatisfactory results and in order to produce atolerable number of voids, more than one gate needs to be used.

In a second embodiment of this invention the steering wheel rim iscovered as shown in FIG. 7. The wrapped steering wheel may be anintegral steering wheel as shown in FIGS. 2 and 3 or may be anon-integral steering wheel 10 a as shown in FIGS. 5 and 6. Thecomponents of FIGS. 5 and 6 correspond to like numbered components ofthe integral steering wheel shown in FIGS. 2 and 3. It is to beunderstood that the steering wheels shown in FIGS. 2 and 5 are onlyexemplary of such items and that the structure, appearance, anddimensions may be varied in accordance with good engineering practiceswithout deviating from the scope of the present invention. The steeringwheel 10 a shown in FIGS. 5 and 6 has an airbag cover 7 a separate fromthe rim 8 and the spokes 9. The airbag 11 is installed between theairbag cover 7 a and the hub 2. FIG. 7 shows the rim portion 8 coveredwith a cushion 110 and a wrap 112. In a preferred embodiment, an outsidewrap 112 is attached to a cushion 110 by a wrap-cushion adhesive 114 andthe cushion 110 is attached to the rim portion 8 with a substrateadhesive 116. Many various types of adhesives, cushions, and wraps maybe used in accordance with good engineering practice without deviatingfrom the scope of this invention. In a certain preferred embodiment, aleather wrap 112 is attached to a cushion foam 110 such as Rubatex 1400Nwith a neoprene adhesive 114, such as Midwest 23-20-1, and the cushion110 is attached to the rim with an urethane adhesive 116 such as U10FLmanufactured by Loctite. However, it may be understood by a personskilled in this art that many variations may be made without deviatingfrom the scope of this invention. For example, the wrap may be attachedto the steering wheel rim without a cushion.

In the past, PVC was used for steering wheel rims to be covered.However, the plasticizer in PVC migrates to the surface, causingadhesion problems with paint or a cover adhered to the rim. When theplasticizer migrates to the surface of a wrapped rim, the plasticizerdissolves the adhesive and is absorbed into the cushion and the wrapmaterial, causing the wrap to soften and degrade.

To solve the above problem a polyester-based thermoplastic elastomer wasused for the steering wheel rim. In a preferred embodiment, DSM's EM400was used because it has good long-term adhesion and no plasticizermigration. However, other thermoplastic elastomers may be used such aspolypropylene. A foaming agent was added to the thermoplastic elastomeror “TPEE” to reduce the hardness and weight of the steering wheelmaterial. In a preferred embodiment, the foaming agent was a proprietaryfoaming agent manufactured by Clariant Chemical, identified as ClariantPRA0698418. The chemical make up of this foaming agent is not knownbecause Clariant Chemical has a proprietary right to the chemical.

The foaming agent consists of a foam, a carrier, and a color. Adhesionproblems existed when the foaming agent contained an olefin carrier ordispersing aid. An olefin carrier may migrate to the surface over timeand cause adhesion degradation to paint or a cover adhered to the rim.Olefin wax melts between 60-120° C. Therefore, olefin wax becomes activeand migrates to the surface of the steering wheel during normal storageand field exposure. To improve adhesion, a foaming agent with anon-olefinic carrier should be used. In a preferred embodiment, ClariantPRA0698418, a foaming agent with a polyvinyl acetate (PVA) carrier, wasadded to EM400 with a black color concentrate, to make a steering wheelrim and airbag. Although the PVA carrier may migrate to the surface itshould not create adhesion problems.

EXPERIMENT 4: ADHESION TESTS

FIG. 8 shows the results of heat aged adhesion tests at 85° C. conductedon plaques of PVC, Polypropylene, and EM400 with foaming agent ClariantPRA0698418. The foamed EM400 was tested with Loctite U10FL adhesive anda Rubatex 1400N cushion foam. Montell's KS359P Polypropylene was testedwith 3M's DP8005 adhesive and Rubatex 1400N cushion foam. Two PVCplaques were tested. First, a PVC plaque with 3M 924 tape and a Griswold3120 cushion foam was tested. Second, a PVC plaque with Loctite adhesiveand a Rubatex 1400N cushion was tested. The PVC plaques had an initialpeel strength of 0.5 pounds/inch width and no peel strength after ashort period of exposure. In comparison, both thermoplastic elastomershad superior adhesion, showing relatively minimal degradation over 1000hours of heat aged exposure.

While the forms of the apparatus and method herein described constitutepreferred embodiments of the invention, it is to be understood that thisinvention is not limited to these precise forms of apparatus and thatchanges may be made therein without departing from the true spirit andscope of the invention which is defined in the appended claims.

We claim:
 1. A steering wheel comprising an armature having a hubportion connected to a rim portion by a spoke portion; the rim portionand spoke portion of the armature each acting as a direct supportstructure for a single thermoplastic foam material which has acontinuous external surface around the rim portion and spoke portion andforms an airbag cover which is aligned with and spaced apart from thehub portion of the armature, the portion of the thermoplastic foammaterial supported by the rim portion has different physical propertiesthan the portion of the thermoplastic foam material forming the air bagcover; and a wrap covering the portion of the thermoplastic foammaterial supported by the rim portion.
 2. The steering wheel of claim 1wherein the wrap is leather.
 3. The steering wheel of claim 2 furthercomprising a cushion positioned between the wrap and the theromoplasticfoam material.
 4. The steering wheel of claim 1 further comprising acushion positioned between the wrap and the thermoplastic foam material.5. The steering wheel of claim 1 wherein the thermoplastic foam materialis formed of a foaming agent with a nonolefinic carrier added to athermoplastic elastomer.
 6. The steering wheel of claim 5 wherein theportion of the thermoplastic foam material supported by the rim portionhas at least a ten percent lower density than the thermoplasticelastomer.
 7. The steering wheel of claim 1 wherein the thermoplasticfoam material is a foamed polypropylene.
 8. A steering wheel comprisingan armature having a hub portion connected to a rim portion by a spokeportion, an airbag cover aligned with and spaced apart from the hubportion, wherein a thermoplastic foam material surrounds the rim portionand forms the airbag cover, the thermoplastic foam material comprises athermoplastic elastomer, the same thermoplastic elastomer surrounds therim portion and forms the airbag cover, the portion of the thermoplasticfoam material surrounding the rim portion has different physicalproperties than the portion of the thermoplastic foam material formingthe air bag cover, and a leather wrap adhered to the thermoplastic foammaterial.
 9. The steering wheel of claim 8 further comprising a cushionpositioned between the thermoplastic foam material and the leather wrap.10. The steering wheel of claim 8 wherein the thermoplastic foammaterial further comprises a foaming agent having a non-olefiniccarrier.
 11. The steering wheel of claim 10 wherein the foaming agentcarrier is polyvinyl acetate.
 12. The steering wheel of claim 8 whereinthe thermoplastic foam material is a foamed polypropylene.